2019-06-01_Motorcycle_Mojo_Magazine

62 MOTORCYCLE MOJO MAY 2019

MOJO GARAGE

tech by Costa Mouzouris

Synchronizing

Carburetors

lthough carburetors have

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cient fuel injection on modern

motorcycles, there are still

plenty of older, carburetted

bikes out there. Carburetors

require some periodic adjustments,

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occasionally on multi-cylinder engines

is carburetor synchronization. On

bikes that have more than one cylinder,

synchronizing the carburetors allows

the cylinders to work in unison as the

throttle is opened, thus allowing all

cylinders to operate with the same

output (some V-twins use only one

carburetor, so this is unnecessary). The

engine therefore operates smoothly and

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out of sync will vibrate, possibly lack

power, and sound, well, off.

The ideal way to synchronize

carburetors is by using a synchronizing

tool, which plugs into the individual

intake manifolds and measures intake

manifold vacuum as the engine runs

and the throttle is operated. The tool

will display the vacuum level for each

cylinder simultaneously, either on a set

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transparent tubes. If the throttles are

not set equally, the vacuum will not

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Syncing carbs is a must for a smooth

running multi-carb engine

DIFFICULTY LEVEL:

read evenly; the vacuum will be higher

on the cylinder or cylinders on which

the throttle valves are slightly closed,

compared to the cylinders with lower

vacuum readings. The more the throttle

is closed for a given engine speed, the

higher the intake vacuum will be.

Adjusting the throttle valves varies

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tion. If the individual carburetors are far

apart, as on a boxer twin, the adjuster

will be at the throttle cable going to each

carb. Parallel-twins and inline-fours have

individual carburetors lined up side by

side, with screw-type adjusters connecting

the throttle valves between adjoining

carburetors. Some older slide carburetors

have adjusters atop each slide.

If you don’t have a synchronizing

tool, you can still synchronize carbure

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tors accurately, though to do so, it’s best

to remove the carburetors from the bike

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or mechanical slides from the rear. It’s

important to note that you must use

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since the vacuum slides don’t move

unless the engine is running.

On a twin, there will be one adjusting

screw; on a four cylinder, there will be

Usually the number two carb is the “base” carb and the other carbs are adjusted after this one is properly set.

The liquid in the tubes

Àuctuates as the carbs are

adjusted giving an accurate

vacuum reading.